In digital electronic systems, data is often transmitted across transmission lines. On such transmission lines, the most efficient transfer of data from a transmitter to a receiver at the end of the line occurs when the terminator impedance matches the cable impedance. "Matching" occurs when the impedance of the terminator equals the characteristic impedance of the transmission line. Transmission line terminators are connected to the ends of transmission lines to provide for impedance matching. Existing terminators for transmission lines, such as small computers systems interface ("SCSI") interface cables, have significant limitations.
In typical transmission lines, such as the SCSI interface, a transmitter will assert the transmission line or deassert the transmission line. By asserting the transmission line, the transmitter may pull the transmission line to a particular voltage, such as 0.5 volts. By deasserting the line, a transmitter releases the line, allowing it to rise to a high voltage, which, in SCSI systems is about 2.85 volts. Transmission line terminators are designed to insure that the voltage levels associated with these assertions and deassertions are received by receivers along the transmission line. In particular, transmission line terminators supply current to the transmission line when the transmission line is pulled to a particular voltage, for example 0.5 volts, and should act as voltage sources when the transmission line is driven to a particular voltage, such as 2.85 volts.
FIG. 1 illustrates the I-V characteristics of transmission line terminators. As shown by curve 1, representing the ideal terminator, a maximum terminator current is supplied until the voltage reaches a particular level. When that voltage level is reached, the terminator should act as a voltage source and provide zero current. As an example, in typical SCSI applications, the high output voltage ("V.sub.oh ") is approximately 2.5 volts, and a somewhat higher voltage, for example 2.85 volts, is the open circuit voltage ("V.sub.oc "), with receivers registering a high output voltage at about 2.0 volts. Thus, until the transmission line cable reaches 2.85 volts, an ideal terminator would supply maximum current. Once the transmission line reaches 2.85 volts, the terminator should behave as a voltage source to maintain the voltage level.
Existing transmission line terminators provide for resistive termination. For example, one approach uses two resistors in series connected between a reference voltage and ground, with a typical reference voltage of 4.75 volts and resistor values of 220 ohms and 330 ohms (known as a 220/330 resistor terminator). The transmission line is terminated between the two resistors. In this manner, current is supplied from the reference voltage to the transmission line until the voltage on the transmission line reaches the open circuit voltage V.sub.oc. FIG. 1 illustrates the I-V characteristic of such a terminator. As can be seen from this graph, the 220/330 terminator fails to provide relatively large amounts of current over a wide range of voltages.
Another existing approach to resistive termination involves the use of a voltage regulator connected to the transmission line through a resistor, known as a Boulay terminator. In SCSI applications, this resistor has a value of 110 ohms and the voltage regulator would have a voltage of approximately 2.85 volts. The I-V characteristic of this terminator is shown in FIG. 1. As can be seen, this terminator also fails to supply relatively large amounts of current for a wide range of voltages on the transmission line.
Therefore, a need has arisen for a non-linear transmission line terminator capable of outputting relatively constant amounts of current over a wide range of transmission line voltages.